The present application relates to electroscopic display devices and, more particularly, to a novel electroscopic display device having the electrode structure thereof fabricated upon a single substrate.
Electroscopic display devices are known to the art. Several such devices, and methods for the fabrication thereof, are disclosed and claimed in my co-pending U.S. application Ser. No. 303,275, filed Sept. 17, 1981, assigned to the assignee of the present invention and incorporated herein by reference in its entirety. The electroscopic displays described therein comprise moveable metal plates in a fluid-filled cell. This form of electroscopic display requires that each metal plate be capable of movement, from a resting position adjacent a first substrate to an active position adjacent a second substrate spaced from the first substrate, through a fluid layer, typically having a thickness on the order of 15 microns. Hitherto, electroscopic display devices have been built with the conductive plate fabricated upon the interior surface of one of the substrates (e.g. the rear substrate interior surface) with a stationary electrode fabricated upon the interior surface of the remaining substrate (e.g. the front substrate interior surface). For example, in FIG. 1, prior art display cell 1 utilizes a pair of spaced transparent substrates 2 and 3, upon the interior facing surfaces of which are respectively fabricated stationary electrode 4, overcoated with a thin dielectric layer (not shown), and an electrode-plate structure including conductive portions 5 fixed to the interior surface of substrate 3 and moveable plate conductor 6, connected by spring member 7 to fixed portions 5. A light-absorbing fluid layer 8 fills the volume between the substrates. A voltage source 9a is connected in series with switching means 9b, between the front electrode 4 and the fixed rear electrode-plate portions. With switch 9b open, a field does not form between electrodes 4 and 6 and the springs 7 cause plate 6 to rest against the interior surface of substrate 3 (as shown at the left side of FIG. 1). Light enters through substrate 2, is passed through substantially transparent electrode 4 and then is essentially absorbed in liquid 8; the area defined by plate 6 is relatively dark. Upon closure of switch 9b, a voltage is applied between electrode 4 and plate 6' , and plate 6' is attracted upwardly to plate 4 (as shown at the right side of FIG. 1). Plate 6' having moved to a position adjacent electrode 4, light entering through substrate 2 is transmitted through electrode 4 and reflected by plate 6' after undergoing relatively little attenuation in liquid 8; the area defined by plate 6' is a viewable bright area through substrate 2. Upon opening switch 9b, the field is removed and the energy stored in springs 7' return plate 6' to a rest position adjacent rear substrate 3.
It will be seen that any variation in the spacing S between the front electrode and rear electrode-plate portions will cause a variation in the voltages required to turn "on" and turn "off" the cell element defined by a particular plate 6. This turn-on and turn-off voltage variation problem is particularly noticeable in large area displays, and especially in such displays utilizing a relatively low cost glass for the substrates. It is therefore highly desirable to provide an electroscopic display device having display cell operating characteristics which are independent of display cell thickness.